Absolutely. If you don't like it, sell them weed and then
  you have their $500.

Except now, you've used your time and resources to produce weed instead of other goods and services. The people who would have used that $500 to buy those other goods and services will subsequently have a lower standard of living.

If someone steals $5k from me and uses that $5k to buy my car, am I just as well off as I was before because I still have my $5k?